Electron microscopy, biochemistry and physiology will be combined in an effort to identify the lesion or lesions which develop in platelets stored in vitro at 22 degrees Centigrade, 4 degrees Centigrade, minus 80 degrees Centigrade, and minus 150 degrees Centigrade. Changes in levels of serotonin and serotonin metabolites, adenine nucleotides, adenylate energy charge, lipid peroxides, malondialdehyde, cAMP, and cGMP and hydrolytic enzymes will be correlated with ultrastructural changes and difference in response to a variety of aggregating agents. Special studies on the microtubules and contractile proteins of stored platelets will be conducted. Chemical agents which influence the structural and biochemical elements of platelets will be employed in an effort to circumvent the storage lesion. The investigation will provide information essential for long term storage of platelets in vitro.